As is known, more recent alpine or Telemark ski boots are made up of a rigid shell made of plastic material and which is shaped so as to accommodate the foot of the user, and is inferiorly provided with a front sole and with a rear heal usually made of non-slip elastomeric material; of a rigid cuff made of plastic material and which is C-shaped so as to surround the ankle of the user from behind, and is hinged to the upper part of the shell so as to rotate about a transversal reference axis substantially coinciding with the articulation axis of the ankle; of an inner liner made of soft and thermal insulating material and which is removably inserted into shell and cuff, and is shaped so as to surround and protect both the foot and the lower part of the leg of the user; and of a protective upper tongue made of plastic material, which has its lower end hinged to the shell, immediately above the tarsal-phalangeal area of the foot, and extends along the upper part of the shell up to beneath the cuff, so as to cover the longitudinal slit of the shell which allows the shell to be temporarily widened in order to facilitate the insertion of the foot into the liner.
The rigid shell of alpine and Telemark ski boots is also frontally provided with a substantially duckbill-shaped, protruding appendix, which protrudes from the ogival tip of the shell while remaining locally substantially coplanar to the front sole, and is structured so as to be firmly and stably coupled, though easily releasable manner, to the toepiece of the ski fastening device which in turn is firmly attached on the middle part of the downhill ski.
In the practice of Telemark skiing, it is also essential to be able to significantly lift the heal of the ski boot from the ski below, while always keeping the tip of the boot firmly anchored to the ski, whereby the front part of the shell of Telemark ski boots is provided at the top with a programmed-deformation gusseted folding which straddles the metatarsal area of the foot, so as to allow the front part of the shell to bend forwards with respect to the tip, so as to go along with the natural movement of the skier's foot.
The ski fastening device is structured instead so as to go along with both the deformations of the shell as a result of the bending of the skier's foot, and with the cyclical lifting of the boot heal from the ski below, thus always and in any event keeping the tip of the boot, or better the duckbill-shaped appendix of the shell, firmly anchored to the ski.
More in detail, the toepiece of the most recent ski fastening devices is structured so as to fix the boot above the ski, tightening the shell at the front sole or solely at the duckbill-shaped protruding appendix, thus allowing the shell to rotate, with respect to the ski beneath, about a horizontal reference axis which is arranged immediately close to the duckbill-shaped protruding appendix and is locally perpendicular to the center line plane of the boot and to the center line plane of the ski.
Unfortunately, recent studies on the kinematics of the movement of the alpine and Telemark ski boot on the ski have shown that, during the initial part of the lifting movement of the boot heal from the ski, the ski fastening devices are not able to transfer the push of the skier to the ski beneath, thus resulting in a part of the skier's movement not implying a forward push.